1. FIELD OF THE INVENTION
This invention lies in the field of the burning of gaseous fuels in furnaces and the like. More particularly, it concerns the burning of gaseous fuels, where the fuels have a wide range of molecular weight, and corresponding calorific value, and wherein the full amount of combustion air is inspirated by the kinetic energy of the fuel flowing to the burner.
2. DESCRIPTION OF THE PRIOR ART
In the prior art it has been customary to build a furnace burner system which is designed substantially entirely on the basis of the use of a single fuel. Thus, given the single fuel, namely one which has a preselected molecular weight and calorific value, the burner system is designed so that the applied pressure of the fuel, size of orifices, etc., are such as to inspirate the full amount of combustion air, required to maintain a minimum value of excess oxygen in the products of combustion as they flow to the stack.
Those who are versed in the art of design and operation of fuel gas burners consider it axiomatic that there is very little room for error in either the design or operation of fuel gas burners which inspirate 100% of their air requirement for theoretical air, plus an optimum excess air factor which is typically indicated by from 1% to 2% oxygen in the combustion gases directly enroute to a stack or chimney for venting to atmosphere after suitable heat extraction from them.
Burner design must be very closely controlled, but those skilled in the art of burner design, who have established practices for such control, find little difficulty. Thus, this factor is under control. But an unescapable factor as relates to burner operation is that such burners are severely limited for acceptance of varying gas fuels, where there is increase in fuel calorific value and fuel molecular weight. An upper limit for calorific value increase without upset, is in the order of 5% as relates to BTU/cubic feet.
This limitation prevents the use of a number of available fuel supplies, and necessitates the use of a fixed gaseous fuel supply (which is natural gas in a preponderance of cases) to seriously interfere with fuel heat energy conservation in areas where the conservation is most needed.
Reasons for this are complex, but they do require understanding. Burner air inspiration results from use of the energy made available as the gas fuel is discharged from supply pressure (which is generally as much as 15 gauge) at critical or sonic velocity, from an orifice. The orifice discharges gas fuel coaxially into an aspirator throat. Air is drawn into and mixed with gas so the discharge from the aspirator throat, at its down stream end, is a burnable gas-air mixture with a selected quantity of excess air for the gas fuel. Oxygen at 1% in the combustion effluent gases indicates substantially 5% excess air, and Oxygen at 2% is substantially 10% excess air. Efficiency of fuel-supplied heat usage is maximum at lowest air. O.sub.2 is an accurate indicator of excess air (rather than CO.sub.2), and for that reason, in any case of fuel burning, it is preferable to monitor the O.sub.2 content of the effluent combustion gases as nearly constantly as possible because, in any case of fuel burning for production of useful heat, the efficiency of heat utilization is according to the excess air present as the fuel burns. Higher excess air denotes fuel wastage, and lower excess air denotes fuel conservation.
It is axiomatic among well-informed operating people to accept 1% O.sub.2 -5% excess air as an absolute minimum, and 2% O.sub.2 -10% excess air as a preferred maximum figure which, while it is optimum, should be checked (monitored) as nearly constantly as possible. Such monitoring is increasingly present in industry, but far from universally present. The use of 100% inspirating burners is typically (but not necessarily) limited to highly pyrophoric processes such as ammonia synthesis or hydrocarbon `cracking` for preferred olefins production because of the precise process control they provide when suitably controlled, and supplied with suitable gaseous fuels.
But the stringent limitation which applies to `suitable fuels` has, in prior practice, demanded the use of fixed fuels rather than a variety of fuels, such as are typically found in process facilities to, at times, result in fuel wastage. As has been previously noted, increase of 5% for fuel calorific value per cubic foot has, heretofore, been a limiting factor in fuels usage.
This invention, in differentiation from the prior art, allows the use of fuels in which the molecular weight and calorific value may increase by as much as 100%, while maintaining a very stable operation, with the same burner structures which formerly were intolerant of more than a 5% increase in molecular weight or calorific value.
All fuel gas burners are not designed for supply of all their air for combustion through gas-inspiration of air, as primary air or air premixed with the gas fuel. They rely on additional air supply for make-up of total air for combustion demand. The second (secondary air) supply of air may be due to furnace draft or other means for air supply to furnaces, and is controlled as to quantity by any of well-known devices common to the art of burning fuels. But there is always a fixed ratio of primary to secondary air volume (quantity) for supply of total air demand, if optimum (best) fuel conservation is to be observed, and stack gas O.sub.2 remains constant.
The secondary air volume is firmly fixed by the secondary air supply means and control, but the inspirated primary air volume is according to gas-supplied discharge energy, as is the case with 100% inspirating burners. Thus, and as the calorific value (molecular weight) of the gaseous fuel changes, the volume of inspirated primary air changes to destroy the primary-secondary air ratio, which will be productive of a preferred O.sub.2 concentration in the effluent combustion gases.
This ratio must be maintained for greatest fuel conservation. For this reason, preheat of the gaseous fuel to a selected temprature can maintain gaseous fuel energy for air inspiration, as per the table on Page 9, for burners which aspirate all combustion air, and also to maintain the ratio of primary to secondary air, which is productive of a preferred O.sub.2 content in the effluent combustion gases, in burners which require both primary and secondary air for their burning of fuels.